The present invention relates to a method of monitoring the diameter of columns made by the technique of injection under pressure known as xe2x80x9cjet groutingxe2x80x9d.
That technique consists in drilling a substantially cylindrical vertical hole in the ground by using a tool which delivers one or more jets of liquid under pressure enabling the ground installation to be prepared, with the hole obtained in this way being filled with a grout or analogous material so as to obtain a column which is molded in the borehole. That technique is described in particular in French patent No. 2 700 128.
It will be understood that during the operation of xe2x80x9cdrillingxe2x80x9d the hole by means of jets of liquid. Under pressure, the actual diameter of the borehole can vary significantly at successive depths, depending on the irregularities encountered in the subsoil, and in particular depending on the varying nature of the subsoil. As a result, the column obtained in this way can have significant relative differences of diameter depending on the depth under consideration. This problem is particularly severe when the depth of the column is large, which depth can be as much as 15 meters.
In some cases, a succession of adjacent columns are made so as to constitute a continuous barrier in the ground. It is clear that for that type of application, it is particularly important to be able to monitor the diameter of each column at different depths so as to be sure that the resulting barrier is continuous over its entire height.
At present there is no method of monitoring the diameter of columns made by jet grouting once they have been made. The technique used consists in making a test column whose top portion is unearthed so as to inspect its diameter visually. It will nevertheless be understood that that technique is relatively burdensome to use and in any event does not enable the diameter of the deeper portions of the column to be monitored.
Techniques are also known for making electrical measurements in order to determine the nature of the ground to a certain depth. In those techniques, a probe comprising a succession of regularly spaced apart electrodes is installed in a borehole, and the electrodes serve both as electrodes for measuring potential difference and as electrodes for injecting electricity into the ground.
The electrodes that inject electric current create electric field lines in a cylindrical volume surrounding the borehole, and the potential difference measurements performed at different depths in the borehole enable the characteristics of the ground to be determined at different depths by measuring a physical magnitude representative of the nature of the ground. This physical magnitude is constituted by the resistivity of the ground which, when determined, can be used to evaluate the nature of the ground. One such technique is described in detail in European patent No. 0 518 686, in the name of the Applicant.
An object of the present invention is to provide a method of monitoring the diameter of columns made in the ground by jet grouting, which method is not of the destructive type, and enables monitoring to be performed over the full height of the column made.
To achieve this object, the invention provides a method of monitoring the diameter of columns made in the ground by jet grouting, the method comprising the following steps:
a measuring instrument is provided that is constituted by a tubular element of length substantially equal to that of the column, said tubular element being fitted with a plurality of emission electrodes for creating an electric field and with a plurality of measurement electrodes for measuring potential differences created by said electric field, the electrodes being connected respectively to a source of electricity and to potential measuring means;
a reference borehole is made in the ground close to the location where the columns are made, and said measuring instrument is placed therein to take voltage measurements representative of the physical characteristics of the ground at different depths of the reference borehole so as to obtain a series of reference measurements for said depths;
an axial borehole is bored in the column to be tested, with the diameter of the axial borehole being much smaller than the diameter of said column;
said measuring instrument is inserted into the borehole made in said column and voltage measurements are made corresponding to different depths, thus obtaining actual measurements of voltages representative of a physical characteristic associated with said column and with the surrounding ground at different depths; and
said actual measurements are processed with reference to said reference measurements so as to obtain information representative of the diameter of said column at said different depths.
It will be understood that in this method, a reference borehole is made initially close to the location where the columns are to be made. Electrical measurements are taken in the reference borehole to obtain the reference physical magnitudes corresponding to different depths in the resulting borehole. The depth of the borehole naturally corresponds to the depth of the columns to be made.
In a second step, after the column has been made by jet grouting, an axial borehole is made in the resulting column and the same measuring apparatus is used to take measurements at different depths of the borehole, and thus of the column. The measurements taken in this way serve to obtain, for each depth, a physical parameter which is associated firstly with the diameter of the column at said depth and secondly with the surrounding portion of ground involved in the electric field that is created. By processing these measurements at each depth in association with the results obtained when performing reference measurements in the reference borehole, it is possible to deduce values which are inherent to the column on its own, and thus to deduce the diameter thereof or at least a relative magnitude representative of variations in the diameter thereof.
In a first implementation, while boring the axial borehole in said column, any inclination of said borehole relative to the vertical is measured at different depths so as to obtain a series of inclination measurements, and the information representative of column diameter for the different depths is corrected with the help of said inclination measurements.
In this improved implementation, it will be understood that account is taken of any inclination of the borehole made in the column prior to installing the measuring instrument. Determining inclination at different measurement depths enables the actual measurements to be corrected and thus enables diameter measurements to be corrected.
In a first implementation, said axial borehole is made when the material for making said column has not yet set.
In a second implementation, said axial borehole is made when the material constituting said column has set, at least partially.
Preferably, the physical magnitude is the resistivity of the ground or of the material constituting the column, and in order to process said actual measurements with the help of said reference measurements, special software is used for providing a three-dimensional interpretation of the reference resistivity measurements and of the actual resistivity measurements relating both to the shape of the column and to the surrounding ground.